As an auto focusing (AF) method for an image-pickup apparatus such as a digital single-lens reflex camera, a so-called Through The Lens (TTL) phase-difference detection method has been known. In a camera using the TTL phase-difference detection method, light coming through an image-pickup lens is separated by a light-separating member such as a mirror and transmitted light is guided to an image-pickup system and reflected light is guided to a focus detection system.
As described above, in the camera using the TTL phase-difference detection method, the image-pickup system and the focus detection system are separately provided. This causes a problem as described below.
In the case of a general silver halide film, the image-pickup system generally has the highest spectral sensitivity characteristics to light of about 400 to 650 nm in order to provide the color reproducibility suitable for characteristics of human eyes.
On the other hand, silicon photo diode constituting an image-pickup device such as a CMOS sensor used for the image-pickup system generally has a sensitivity peak of about 800 nm and has the sensitivity up to about 1100 nm at the long-wavelength side.
However, in order to place importance on color reproducibility, light having a wavelength beyond the frequency range is blocked, causing some sacrifice in the sensitivity.
In the case of a photoelectric conversion device using the phase-difference detection method that is a sensor performing a focus detection, the sensitivity is generally up to about 1100 nm.
However, many photoelectric conversion devices have the sensitivity higher than 1100 nm by 100 nm in order to perform the focus detection even to a low luminance object and to allow a camera to project AF assist light in a near-infrared region (about 700 nm) under a low luminance to an object to perform an accurate focus detection.
FIG. 9 shows light-dividing sensitivities of various light sources, the image-pickup device, and the assist light. The horizontal axis represents a wavelength and the vertical axis represents a relative focal point depending on chromatic aberration of relative energy or lens.
In FIG. 9, C denotes the chromatic aberration of the image-pickup lens and B, G, and R denote light-dividing sensitivities of a blue pixel, a green pixel, and a red pixel of a primary-color-type image pickup device, respectively. F denotes a fluorescent light. L denotes a photoflood lamp. A denotes the light-dividing sensitivity of the above-described assist light.
As can be seen from FIG. 9, while the wavelength component of the fluorescent light includes substantially no wavelength components longer than 620 nm, the photoflood lamp shows a higher relative sensitivity toward the longer wavelength side.
On the other hand, the chromatic aberration C of the lens shows a different focal point depending on the wavelength and a longer focal length toward the longer wavelength side.
Thus, when the focus detection sensor having the highest sensitivity at 700 nm is used, the fluorescent light and the photoflood lamp having less long wavelength components cause a difference in detected focal points, causing a focal shift in the image-pickup device.
With regard to the problem described above in which the focal point detected by the focus detection system is shifted depending on the light-dividing sensitivity of the light source, a camera correcting the focal point is disclosed in Japanese Patent Laid-Open No. 2000-275512.
This camera compares outputs of two types of sensors having different light-dividing sensitivities to determine the type of the light source to correct the focal point to correct the focal shift due to the light-dividing characteristic.
Japanese Patent Laid-Open No. 62-174710 discloses a method in which the chromatic aberration amount of an interchangeable lens is stored in a memory in the lens and the defocus correction amount is calculated by multiplying a predetermined coefficient with the lens chromatic aberration amount based on the determination result of the type of the light source.
However, in the case of the auto-focusing cameras disclosed in Japanese Patent Laid-Open No. 2000-275512 and Japanese Patent Laid-Open No. S62-174710, a problem is caused where, when the type of the light source is determined while projecting the AF assist light, the focal point may be corrected in a wrong manner.
The focal shift due to the light-dividing wavelength when the AF assist light is projected will be described with reference to FIG. 10 and FIG. 11. FIG. 10 shows a relationship between a contrast pattern of the AF assist light and a position of the view field of the AF sensor (AF view field).
FIG. 11 shows pixel information obtained by the AF sensor when the AF assist light of FIG. 10 is projected. The horizontal axis represents a pixel position and the vertical axis represents the signal intensity of a pixel.
It is assumed that there is no contrast of an object and there is no contrast of pixel information only due to ambient light. The ambient light is assumed as light other than illumination light (AF assist light) from the camera side.
The AF assist light projects a predetermined contrast pattern light on ambient light and thus the AF assist light forms a contrast in pixel information. AF is performed based on this contrast.
In other words, when there is no object contrast or a low object contrast, the detection of a defocus amount is performed based on the contrast by the AF assist light. Thus, a focal shift due to the wavelength of only the AF assist light is caused.
Thus, in the case described above in which there is no object contrast or a low object contrast and the AF assist light is projected, the determination of the type of the light source must be subjected to the correction based on the wavelength of only the AF assist light except for the ambient light.
However, in the case of the auto-focusing cameras disclosed in Japanese Patent Laid-Open No. 2000-275512 and Japanese Patent Laid-Open No. S62-174710, the operation for the determination of the light source when the AF assist light is projected is not taken into consideration.
Furthermore, the auto-focusing cameras also cause, when the type of the light source is determined while projecting the AF assist light, the light source mixed with not only the AF assist light but also the ambient light to be determined, causing a wrong focal point correction.
The present invention provides an image-pickup apparatus, an image-pickup system, and a method for controlling an image-pickup apparatus by which a highly-accurate AF control can be performed under various light sources including the AF assist light.